1. Field of the Invention
The present invention is directed to the field of semiconductor manufacturing and more specifically to the area of ion dose measurements performed during the manufacturing process.
2. Description of the Prior Art
Ion implantation is a common semiconductor processing step used to modify the near-surface chemical and physical composition of a wafer by the injection of high energy ions. The crystal lattice of the semiconductor is damaged by the ion injection process. Normally, the ion beam is small (5 mm), compared with the size of the wafer (75-200 mm). Therefore, the scanning of the ion beam with respect to the wafer is necessary to achieve a uniform ion dose. Variation in ion dose may occur over the wafer surface due to a number of factors including improper scanning, wafer charging or beam instability. Of course such variation in the ion dose will give rise to variation in the electrical properties of the semiconductor device being fabricated. Variation in the ion dose then, in turn, has a direct impact on the quality yield. Accordingly, techniques have been developed that give an estimation as to ion dose and in some cases the uniformity of the dose.
One of the known measurement techniques involves direct sheet resistance. In that method, the wafer is subjected to a high temperature annealing step in order to eliminate lattice damage and to activate the implanted ions. Subsequently, a collinear four point probe is placed into electrical contact with the wafer surface and current is forced through two of the probes. Voltage is then measured across the other two probes and a resistance reading is made.
Another method utilizes thermal waves that are propagated into the wafer. In that method, high frequency pulses of laser energy are directed to be incident on the semiconductor surface and establish a train of thermal waves that propagate into the wafer. The surface temperature of the wafer is modulated as these waves are scattered by subsurface, implant induced damage. The wafer surface temperature is monitored by the reflection of a second laser beam, thus giving a signal related to ion dose.
An indirect method of measuring ion dose is termed optical dosimetry. In that method, a glass wafer coated with a photo-resist layer is used in the implanter as a dummy test piece. In that test piece, high implantation results in darkening of the photo-resist layer. As a result, the transmission of collimated light directed through the photo-resist is affected and may be measured as a relation of the ion dose.